Technical Field
The present invention relates generally to systems and methods for measuring passive intermodulation (PIM).
Related Art
Passive intermodulation (PIM), resulting from the non-linear mixing of two or more frequencies in a passive device such as a connector or cable, can be problematic for operators of wireless communication systems. PIM can appear, for example, as existing equipment ages, when new carriers are co-located, when new carriers are overlaid (diplexed) into old antenna runs, and when new equipment is installed. In a cellular network, PIM creates interference that can reduce a receive sensitivity of a cell or block calls. This interference can affect the cell that creates it, as well as other nearby receivers. PIM can be a serious issue for operators of wireless communication systems wanting to maximize a system's reliability, data rate, capacity, and return on investment.
High-speed digital data communications have further increased the problematic nature of PIM. PIM has surfaced as a problem for wireless data transmission using modern standards including Long-Term Evolution (LTE), Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) standards. As cell usage and throughput grow, the peak power produced by new digital modulations increases dramatically, potentially contributing heavily to PIM problems. On-site experiments have shown significant decreases in download speeds linked to slight increases in PIM. For example, drive tests have revealed an approximate 18% drop in download speed when residual PIM level is increased from −125 dBm to −105 dBm.
PIM can be reduced or eliminated by replacing or correcting sources of PIM, including faulty cables or connectors. Test systems can be utilized to detect PIM in a communication system and enable a technician to locate the source of the detected PIM. A test system to measure PIM will typically create signals at two (or more) different frequencies, amplify them and provide them, for example, through cables connecting base stations to antennas for transmitting and receiving signals. A return signal carrying PIM is filtered to select a desired test frequency harmonic where PIM is detectable, and measurements of the magnitude of a PIM and the distance-to-PIM are provided to the technician of the test system.